I is the ninth letter of the alphabet. It can also refer to:

I (pronoun), the first-person singular pronoun

In astronomy:

''i'', the symbol for orbital inclination

I, in the Yerkes spectral classification scheme the symbol for supergiants

In chemistry:

I, the symbol for iodine on the periodic table

I, the symbol for isoleucine in biochemistry

In economics:

i, the symbol for the interest rate

I, the symbol for investment

In genetics:

Haplogroup I (mtDNA), a human mitochondrial DNA (mtDNA) haplogroup

Haplogroup I (Y-DNA), a Y-chromosomal DNA (Y-DNA) haplogroup

In mathematics:

1 (number) in Roman numerals

''i'', the imaginary unit for which i² = -1

''I'' (or ''I''_''n''), an identity matrix of indeterminate size (or of size ''n'')

''i'', an index variable in a matrix

''I''_''x''(''a'',''b''), the regularized incomplete beta function (of a variable ''x'' and parameters ''a'',''b'')

''I'', the closed unit interval which contains all real numbers from 0 to 1 inclusive

$\{\backslash hat\{\backslash imath\}\}$ (pronounced "i-hat"), the unit vector along the x-axis in Cartesian coordinates

In music:

I, a diatonic function in tonal music theory

''I'' (album), the debut album of Kurt Nilsen

I (band), a band formed by members of Immortal and Enslaved

''I'' (Cilvaringz album), the debut album of Wu-Tang Clan affiliate producer Cilvaringz

''I'' (EP), an EP by Meshuggah

''I'' (Magnetic Fields album), an album by the Magnetic Fields

''I'', a song on ''Dehumanizer'', an album by Black Sabbath

In physics:

''I'' and ''i'', symbols for electric current signals in electronic engineering

''I'', the moment of inertia

In computer programming:

i, a common generic index variable often used for looping constructs

<i>, an HTML element for marking italic type

.i, the common filename extension for a file that contains the output from a preprocessor.

In radio and television:

i (TV network), the former name of ION Television

I band, the range of radio frequencies from 8 GHz to 10 GHz in the electromagnetic spectrum

I, the ITU prefix allocated to Italy in radio communication

I, in QAM modulation schemes, the in-phase communications channel

''I'', in the YIQ colorspace commonly used with the NTSC television encoding scheme, the color-difference channel which is in-phase with the subcarrier

Other uses:

I (kana), one of the Japanese kana that each respresent one mora

I, the symbol for Italy in the list of international license plate codes

/i/, and , in the International Phonetic Alphabet, the three forms of the letter I

Mitsubishi i, a Japanese ''kei car''

I, a character in Adventure into Fear who acted as a guide for Morbius, the Living Vampire

Lowercase i prefix, a common prefix used by Internet and electronic companies

de:I (Begriffsklärung)
fr:I
nl:I
no:I (andre betydninger)
ksh:I (Watt ėßß datt?)